The moment you realize your hot water is the bottleneck
Most homeowners start thinking about a tankless water heater right after a frustrating moment: the shower goes lukewarm when the dishwasher kicks on, or the kitchen sink takes forever to get truly hot. In San Antonio, TX, that “running out” feeling can happen even with a decent water heater because real life uses overlap more than we think, especially on busy mornings. A properly sized tankless unit fixes that… but an undersized one can leave you with a fancy box on the wall that still can’t keep up.
Tankless sizing is really about matching your home’s peak hot-water demand, not guessing by square footage or picking the biggest unit you see online.
What a tankless water heater actually has to do in your home
A tankless water heater heats water on demand. Instead of storing hot water in a tank, it fires up when you open a hot tap and heats the water as it flows through the unit. That flow is measured in GPM (gallons per minute), which is simply how much water you’re asking the heater to warm at one time.
The other half of sizing is temperature rise, meaning how many degrees the unit has to raise the incoming water to reach your set temperature. If the water comes in cool and you want it hot, the heater has to work harder, and the GPM it can support drops.
So when you’re asking “what size do I need,” you’re really asking: How many gallons per minute of hot water do I need at my busiest moment, at the temperature I want?
Why homeowners end up with cold bursts, weak flow, or buyer’s remorse
Most tankless problems aren’t because tankless is “bad.” They’re usually sizing or installation mismatches that show up once your house starts using the system like normal.
One common issue is sizing based on a single fixture, like “I only care about showers,” and forgetting that showers rarely happen alone. Another is assuming the new tankless will fix low pressure. It won’t. Low pressure is a supply or piping issue, and tankless heaters respond to flow, they don’t create it.
We also see regret when homeowners buy a unit without checking fuel and venting realities. Gas tankless models need adequate gas supply, proper venting, and combustion air. Electric tankless models can require major electrical upgrades. If you’re in an older home, that “simple swap” can turn into a bigger project fast.
And finally, water quality matters. Mineral buildup can reduce performance over time, so your long-term experience depends on maintenance and how your home’s water behaves.
A simple sizing checklist that works in real life
Use this quick checklist to land on a size that actually fits your household:
- Pick your busiest hot-water time of day, like weekday mornings
- List every hot-water use that can happen at the same time (showers, sinks, laundry, dishwasher)
- Find or estimate the flow rate for each fixture and add them together to get total GPM
- Decide your target hot-water temperature (many homes set 120°F for comfort and safety)
- Consider how cool incoming water can get during winter, because more temperature rise means less GPM capacity
- Choose a unit that meets your total GPM at that temperature rise, not just the “max GPM” printed on the box
- Plan for scale control and regular flushing so the unit performs the same next year as it does on day one
The truth about flow, pressure, and “weak hot water” complaints
Flow and pressure get mixed up all the time, so here’s the difference: flow is how many gallons per minute come out of the fixture, while pressure is the force behind it (measured in PSI). You can have high pressure and low flow if an aerator is clogged, a valve is partially closed, or a pipe is restricted.
Tankless sizing is mostly about flow. A helpful shortcut is to look at the fixtures that quietly add up in a home. Bathroom faucets are a big one, and many modern fixtures are designed to limit water use. For example, WaterSense labeled bathroom faucets are designed to use no more than 1.5 GPM. That kind of efficiency changes how much hot water your home demands and can help you size more accurately when you’re adding up a busy-hour scenario like two sinks plus a shower. WaterSense labeled bathroom faucets
Now for the hard truth: if you’ve got a “weak hot side” at one faucet, it’s usually not a sizing problem. It’s commonly a partially closed shutoff valve under the sink, a clogged cartridge, debris in an aerator, or aging galvanized piping restricting flow. A bigger tankless won’t fix any of those.
How hard water changes the sizing conversation in Central Texas
Mineral-rich water can be a quiet deal-breaker for tankless performance if you don’t plan for it. When water contains dissolved calcium and magnesium, it can leave behind scale (that chalky mineral crust) inside plumbing and appliances. Heating water accelerates this, which is why water heaters see it first.
In San Antonio, TX, hard water is a normal reality, and it matters because scale buildup can reduce heat transfer and narrow internal passages over time. That can make a properly sized tankless start acting undersized later, with reduced flow or temperature swings. Texas A&M AgriLife Extension notes that hard water can lead to scale buildup in water heaters and plumbing fixtures, which is exactly the kind of slow performance loss homeowners notice after the “honeymoon phase.” Texas A&M AgriLife Extension notes that hard water can lead to scale buildup
The practical takeaway is simple: when you choose your tankless size, also budget for scale prevention (like softening where appropriate) and regular flushing. Otherwise you’re sizing for day one, not year five.
When the upgrade uncovers hidden plumbing problems
Tankless installations have a way of exposing issues that were already there, especially in older kitchens and utility rooms.
If shutoff valves are seized or half-blocked with corrosion, they may start leaking the moment they’re touched. If supply lines are old or kinked, changing flow patterns can reveal weak spots. If drains are slow, you might suddenly notice gurgling or sewer smells, not because tankless caused it, but because you’re paying more attention and using fixtures differently during a remodel.
A few things you should not do: don’t reef harder on an old valve handle when it won’t turn, don’t overtighten fittings to “stop a drip” (that can crack parts), and don’t ignore water hammer (banging pipes) after installation. Water hammer is a pressure shock in the piping, and it can loosen connections over time.
If you notice persistent gurgling drains, recurring sewer odor, or backups after a plumbing change, it’s worth treating it as a real drainage issue, not just “old house vibes.” Fixing the root cause early is always cheaper than cleaning up cabinet damage later.
How to tell your tankless is working the way it should
A correct install doesn’t just “make hot water.” It should deliver stable temperature, consistent flow, and zero water damage risk.
Start with a basic leak check: look under sinks, behind the unit, and around the water shutoffs. Dry everything with a paper towel and recheck after a few minutes. Next, run two hot fixtures at once and see if the temperature holds steady. If the water swings hot-cold-hot, it can point to undersizing, scale restriction, or a control setting problem.
Over the next few days, do quick cabinet inspections. Look for damp corners, bubbling wood, or that musty smell that shows up before you see visible water. Also listen. A tankless unit should sound like it’s working, but it shouldn’t be rattling, banging, or roaring.
One more smart check: confirm any parts that touch drinking water are certified for that purpose. NSF/ANSI 61 is a key standard used to evaluate materials and products that come into contact with drinking water. NSF/ANSI 61
When calling a plumber is the cheaper option
If you’re confident with tools, tankless still isn’t the best “learn as you go” project. The most expensive mistakes are usually invisible at first: improper venting, undersized gas lines, incorrect condensate drainage, or a setup that technically runs but can’t keep up with real demand.
A pro can also size it based on how your family actually lives. That includes making sure the unit matches your simultaneous uses, your home’s incoming water conditions, and your existing plumbing layout.
If you want help choosing and installing the right unit, PlumbSmart can take care of the full process through our tankless water heater service, including sizing guidance that accounts for your fixtures and your home’s layout. If you’re replacing an older unit and want a clean, code-compliant setup, our water heater installation team can help avoid the common pitfalls that lead to callbacks.
And if hard water has already been rough on your appliances, pairing tankless with a properly designed softening setup can protect the investment long-term. PlumbSmart can help with water softener installation so your new heater keeps performing like it should.
The three actions that make tankless sizing easy
If you remember nothing else, these three steps will get you to the right answer faster than any online “calculator.”
First, base your size on fit, meaning your real simultaneous hot-water uses and your real GPM demand, not a guess or a sales pitch. Second, prioritize reliability, meaning a unit that can deliver that demand at your needed temperature rise without running flat-out every time you shower. Third, plan for your water and piping conditions, especially scale buildup and older shutoffs, so the system stays consistent long after installation day.
Do that, and your tankless heater won’t just be efficient. It’ll be comfortable, steady, and dependable when your household is at its busiest.
